There is a key distinction. In the case of a pre-paid maintenance plan, the maintenance schedule is well known, so the manufacturer / seller can easily assess what their full cost to provide the plan will be, and factoring in the time value of money, determine what an appropriate discount should be for the plan which still is attractive for them to offer.
With a prepaid energy plan, there is no way for Tesla to reasonably estimate what the anticipated total amount of miles you will charge using their network, and in turn, the total consumption of energy will be from their superchargers. So there is no way for them to determine their future expense exposure and be able to price the plan accordingly.
The other key distinction is in the case of the pre-paid maintenance plan, there is nothing that forces any one consumer to actually do the maintenance, or to use the manufacturer for the maintenance. What the pre-paid plan is to lock you in as a customer to actually purchase the service. In that way is actually locks in the future service revenue for them which they might not otherwise receive.
In the case of an energy plan, everyone must purchase energy to use the vehicle. So that future sales stream for the supplier is much more guaranteed, and there is little incentive for them to absorb the future price risk versus just sell to you as you go.
So an apples and oranges comparision.
